This invention relates to aluminum and more particularly it relates to an improved cell for use in the electrolytic production of aluminum from alumina dissolved in a molten salt electrolyte, for example, at low temperatures.
There is great interest in using an inert anode in an electrolytic cell for the production of aluminum from alumina dissolved in the molten salt electrolyte. By definition, the anode should not be reactive with the molten salt electrolyte or oxygen generated at the anode during operation. Anodes of this general type are either comprised of a cermet or metal alloy. For example, U.S. Pat. No. 4,399,008 discloses a composition suitable for fabricating into an inert electrode for use in the electrolytic production of metal from a metal compound dissolved in a molten salt. The electrode comprises at least two metal oxides combined to provide a combination metal oxide.
Also, U.S. Pat. No. 5,284,562 discloses an oxidation resistant, non-consumable anode for use in the electrolytic reduction of alumina to aluminum, which has a composition comprising copper, nickel and iron. The anode is part of an electrolytic reduction cell comprising a vessel having an interior lined with metal which has the same composition as the anode. The electrolyte is preferably composed of a eutectic of AlF3 and either (a) NaF or (b) primarily NaF with some of the NaF replaced by an equivalent molar amount of KF or KF and LiF.
Different processes and electrolytic cell configurations have been suggested for the electrolytic production of aluminum from alumina. For example, U.S. Pat. No. 3,578,580 discloses an apparatus for the electrolysis of molten oxides, especially of alumina, in which the anode is separated from the melt being electrolysed by a layer of oxygen-ion-conducting material, for example cerium oxide stabilized with calcium oxide or other oxides, which is resistant to the melt at the temperature of the electrolysis.
U.S. Pat. No. 4,338,177 discloses a cell for the electrolytic deposition of aluminum at low temperatures and low electrical potential in which the anode is the sole source of aluminum and comprises a composite mixture of an aluminous material such as aluminum oxide and a reducing agent. Conductor means of higher electrical conductivity than the mixture are provided to conduct substantially the entire anodic current to the active anode surface thereby reducing the voltage drop through the highly resistive composite mixture. The mixture may be employed in a self-baking mode or be prebaked. Alternatively, the mixture may be in a particulate form and contained within a porous membrane which passes the electrolyte or other dissolved material while withholding undissolved impurities. The cell may have bipolar electrodes and may be used in combined winning and refining configurations.
U.S. Pat. No. 3,960,678 discloses a process for operating a cell for the electrolysis of a molten charge, in particular aluminum oxide, with one or more anodes, the working surfaces of which are of ceramic oxide material, and anode for carrying out the process. In the process a current density above a minimum value is maintained over the whole anode surface which comes into contact with the molten electrolyte. An anode for carrying out the process is provided at least in the region of the interface between electrolyte and surrounding atmosphere, the three phase zone, with a protective ring of electrically insulating material which is resistant to attack by the electrolyte. The anode may be fitted with a current distributor for attaining a better current distribution.
U.S. Pat. No. 4,110,178 discloses a method and apparatus for producing metal by electrolysis in a molten bath of salt. The apparatus includes an electrolytic cell containing a molten bath of salt and a vertical stack of electrodes located within the bath of salt, with the uppermost electrode being located beneath the upper level of the bath. A baffle extends vertically above the uppermost electrode, the baffle being effective to direct a flow of the bath laterally and beneath the upper level of the bath, and to increase the velocity of the flow of the bath and metal between vertically adjacent electrodes of the vertical stack.
U.S. Pat. No. 4,115,215 discloses a process for purifying aluminum alloys which comprises providing molten aluminum alloy in a container having a porous wall therein capable of containing molten aluminum in the container and being permeable by the molten electrolyte. Aluminum is electrolytically transported through the porous wall to cathode thereby substantially separating the aluminum from alloying constituents.
U.S. Pat. No. 4,243,502 discloses a wettable cathode for an electrolytic cell for the electrolysis of a molten charge, in particular for the production of aluminum, where the said cathode comprises individual, exchangeable elements each with a component part for the supply of electrical power. The elements are connected electrically, via a supporting element, by molten metal which has separated out in the process. The interpolar distance between the anodes and the vertically movable cathode elements is at most 2 cm.
U.S. Pat. No. 4,342,637 discloses an anode for use in the electrolytic deposition of aluminum at low temperatures in which the anode is the sole source of aluminum and comprises a composite mixture of an aluminous material such as aluminum oxide and a reducing agent such as carbon. Conductor means of higher electrical conductivity than the anodic mixture are provided to conduct substantially the entire anodic current to the active anode surface thereby reducing the voltage drop through the highly resistive composite mixture.
U.S. Pat. No. 4,670,110 discloses a process for the electrolytic deposition of aluminum at low temperatures and at low electrical potential in which the anode is the sole source of aluminum and comprises a composite mixture of an aluminous material such as aluminum oxide and a reducing agent. The composite anode is positioned in the electrolyte with at least one active surface of the anode in opposed relationship to but spaced from the surface of the cathode. The greatly increased electrical resistance of the mixture of aluminum oxide and the reducing agent is minimized by passing the anodic current through one or more conductors of low electrical resistivity which extend through the mixture to or approximately to the active reaction face of the mixture in the electrolyte.
U.S. Pat. No. 4,904,356 discloses a carbon block which acts as a cell electrode. Channels are formed in its face which is to face the cell diaphragm. The channels provide an interconnected network including retention pools arranged to hold, release, break up and mix a liquid stream passing through them.
U.S. Pat. No. 5,362,366 discloses an anode-cathode arrangement for the electrowinning of aluminum from alumina dissolved in molten salts, consisting of an anode-cathode double-polar electrode assembly unit or a continuous double polar assembly in which the anode and cathode are bound together and their interelectrode gap is maintained substantially constant by connections made of materials of high electrical, chemical, and mechanical resistance. Multi-double-polar cells for the electrowinning of aluminum contain two or more of such anode-cathode double-polar electrode assembly units. This arrangement permits the removal of reimmersion into any of the anode-cathode double-polar electrode assembly units during operation of the multi-double-polar cell whenever the anode and or the cathode or any part of the electrode unit needs reconditioning for efficient cell operation.
U.S. Pat. No. 5,498,320 discloses a double salt of KAlSO4, as a feedstock which is heated with a eutectic electrolyte, such as K2SO4, at 800° C. for twenty minutes to produce an out-gas of SO3 and a liquid electrolyte of K2SO4 with fine-particles of Al2O3 in suspension having a mean size of six to eight microns. This is pumped into a cell with an electrolyte comprised of K2 SO4 with fine-particles of Al2 O3 in suspension, an anode and a porous cathode of open-cell ceramic foam material. The cell is maintained at 750° C. and four volts of electricity applied between the anode and the cathode causes oxygen to bubble at the anode and liquid aluminum to form in the porous cathode. A channel within the porous cathode, and the porous cathode itself, are deep enough within the cell electrolyte that the pressure head of electrolyte is enough to overcome the difference in density between the molten aluminum and the electrolyte to pump molten aluminum from the channel out of the side of the cell. The electrolyte K2 SO4 is periodically bled-off to control a build-up of the material as aluminum is produced from the double salt of KAlSO4.
In spite of these disclosures, there is still a great need for an electrolytic cell and process for operating the cell at low temperatures that permits efficient electrolytic reduction of alumina to aluminum and removal of molten aluminum without contaminating the aluminum with alumina particles, for example. Further, it is important to remove or drain the molten aluminum from the cathode and collect it in a pool unaffected by turbulence in the bath or molten electrolyte created by evolution of gas such as oxygen at the anode. In addition, it is important to substantially electrically isolate the pool of aluminum in the bottom of the cell from the cathode to avoid parasitic electrical current and losses in current efficiency. The subject invention solves these problems by use of a novel cell and anode which permits efficient removal of molten aluminum from the cathode.